The present invention relates to an apparatus and method for producing antiresonances in a structure at one point when the apparatus is located remotely at another point. More particularly, the invention is concerned with a resonator and method by which an antiresonance is deliberately produced at one point in a structure by tuning of the resonator specially for a remote location in the structure. The data required to properly tune the resonator is obtained from measurements taken from the structure without the resonator.
Vibration reduction in a structure subjected to a continuous vibratory disturbing force at a given frequency is an old and recurring problem especially in vehicular structures such as automobiles, trains, boats and aircraft as well as in some stationary structures in which moving or rotating equipment is housed. One solution that has been used in these situations in the past involves the installation of a vibration absorber at a specified location on the structure where a reduction in the vibration level or an antiresonance is desired. Technically, an antiresonance is a zero vibration level condition which exists or is made to exist at a specified point in a vibrated, undamped structure. In the presence of damping, a zero vibration level will not be achieved; however, a vibration level approaching a zero level can exist or be obtained and is also referred to herein as an antiresonance or antiresonant condition.
In its simplest form, a vibration absorber may be comprised of a mass connected by a spring or other resilient means to the structure at the point where the antiresonant condition is desired. Unfortunately, critical components which require reduced vibration levels for reliable operation or extended life may be clustered in groups which prevent the installation of an absorber at the point of interest. Adequate space for packaging all of the components and the absorber at the same point may not exist, for example, in the cockpit or flight deck of an aircraft where navigation and flight instruments are necessarily clustered within reach or view of the pilot. Obviously mounting an absorber to the exterior of an aircraft at such points is not possible for aerodynamic reasons and mounting the absorber within the flight deck area is impractical, hazardous and virtually impossible as anyone familiar with such crowded areas will understand.
Another method for reducing vibration levels in a structure entails the actual modification of the dynamic characteristics of the structure itself. It will be understood that such solution to a vibration problem, particularly in an existing structure such as an aircraft, is an extreme remedy and is only pursued as a last resort because of the expense involved and the difficulty of predicting precisely what is required by way of a structural change to produce the desired results.
In the article entitled "Application of Antiresonance Theory to Helicopters" by F.D. Bartlett, Jr. and W.G. Flannelly, AHS Journal, Volume 19, No. 1, January 1974 it is shown that at least in cases where a natural antiresonance is close to the point of interest and the frequency of excitation of a structure, an absorber at some other point on the structure tuned to something other than the forcing frequency can produce an antiresonance at the point of interest and the forcing frequency for an excitation at any third point. Unfortunately, the only method know for obtaining this antiresonance was by trial-and-errror testing.
It is, accordingly, a general object of the present invention to provide a method and apparatus by which vibration levels can be reduced in a structure with a remote resonator and without the difficulties of the past .